Add the ability to edit remote directories over SSH (#14530)

This is a first step towards allowing you to edit remote projects
directly over SSH. We'll start with a pretty bare-bones feature set, and
incrementally add further features.

### Todo

Distribution
* [x] Build nightly releases of `zed-remote-server` binaries
    * [x] linux (arm + x86)
    * [x] mac (arm + x86)
* [x] Build stable + preview releases of `zed-remote-server`
* [x] download and cache remote server binaries as needed when opening
ssh project
* [x] ensure server has the latest version of the binary


Auth
* [x] allow specifying password at the command line
* [x] auth via ssh keys
* [x] UI password prompt

Features
* [x] upload remote server binary to server automatically
* [x] opening directories
* [x] tracking file system updates
* [x] opening, editing, saving buffers
* [ ] file operations (rename, delete, create)
* [ ] git diffs
* [ ] project search

Release Notes:

- N/A

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

crates/gpui/src/platform.rs

@@ -64,11 +64,16 @@ pub(crate) use test::*;
 pub(crate) use windows::*;
 
 #[cfg(target_os = "macos")]
-pub(crate) fn current_platform() -> Rc<dyn Platform> {
-    Rc::new(MacPlatform::new())
+pub(crate) fn current_platform(headless: bool) -> Rc<dyn Platform> {
+    Rc::new(MacPlatform::new(headless))
 }
+
 #[cfg(target_os = "linux")]
-pub(crate) fn current_platform() -> Rc<dyn Platform> {
+pub(crate) fn current_platform(headless: bool) -> Rc<dyn Platform> {
+    if headless {
+        return Rc::new(HeadlessClient::new());
+    }
+
     match guess_compositor() {
         "Wayland" => Rc::new(WaylandClient::new()),
         "X11" => Rc::new(X11Client::new()),
@@ -99,7 +104,7 @@ pub fn guess_compositor() -> &'static str {
 
 // todo("windows")
 #[cfg(target_os = "windows")]
-pub(crate) fn current_platform() -> Rc<dyn Platform> {
+pub(crate) fn current_platform(_headless: bool) -> Rc<dyn Platform> {
     Rc::new(WindowsPlatform::new())
 }